Support for blocking FS

> Our current FS assumes every operation is nonblocking, which does not hold for 9p fs over network. To give the illusion that 9p fs is nonblocking, we use spin loop in the `litebox_shim_linux` that waits until an operation is done. This is a temporary workaround that makes it work for the SNP runner, in which case it might be fine because the remote 9p server supposes to run locally at VMPL0. We should not use it elsewhere.

_Originally posted by @CvvT in [#709](https://github.com/microsoft/litebox/pull/709/changes#r2927559565)_

Start by examining the spin loop in litebox_shim_linux and the discussion from pull request #709. Clarify how blocking filesystem operations should behave for 9p over the network and how the SNP runner differs; done means the workaround is not used outside its supported case.

Written by the indexing model from the issue text.